1. Field of the Invention
This invention relates generally to arc fault circuit interrupters, and more particularly to an arc fault circuit interrupter that reliably detects arc faults and rejects electrically similar phenomena, such as noise produced by lamp dimmers, motor controllers, and other random noise sources.
2. Description of the Prior Art
Arc detection is an enhancement to thermal magnetic overload detection, which alone may not detect and respond to arc faults. A number of devices for detecting arc faults and methods of detection have been used in the past. These include the use of E and B field arc sensors, detecting the amplitude of the rate of change of current signals when an arc fault occurs, the use of non-overlapping band pass filters to detect white noise characteristic of arcs, and detecting the disappearance of signals indicating the presence of arcs near zero current crossings. While some of these techniques are more or less effective, they require relatively sophisticated circuits. Heretofore, most arc detection circuits have been incorporated in circuit breakers.
The impedance of electrical circuits to the signals that indicate the presence of an arc fault may make it more difficult to detect arc faults at a circuit breaker than closer to the fault.
There is a need for simple economical arc fault detectors that can be included in wiring devices such as receptacles, plugs or in-line devices, and that offer the same downstream protection as an arc fault detector incorporated in a circuit breaker, but at lower cost. There is a need for an arc fault circuit detector in wiring devices that can be provided at a reduced cost compared with arc fault circuit detecting circuit breakers that is comparable to the reduction in cost between ground fault interrupting receptacles and ground fault interrupting circuit breakers.
Light dimmers are very commonly used in residences where the arc fault circuit interrupters of this invention are likely to be employed. Because light dimmers produce a characteristic noise signature that is similar to the signal produced by some arc faults, it is important for arc fault circuit interrupters to be able to distinguish the signals produced by actual arcs from the signals produced by light dimmers, to avoid false tripping.
It is an object of this invention to provide an arc fault circuit interrupter that employs an electrical circuit that is simple enough, inexpensive enough and small enough to be included in wiring devices. It is another object of this invention to provide an arc fault circuit interrupter that is sensitive to arc faults, but resistant to false tripping, due to noise created by lamp dimmers and the like.
It is another object of this invention to provide an arc fault circuit interrupter that is sensitive to relatively low amplitude series arc faults of at least 5 amps of arc current, typically in series with the load and commonly referred to as Type A arc faults.
It is another object of this invention to provide an arc fault circuit interrupter that detects parallel or line to line arcs, producing currents of 75 amps or more, commonly referred to as Type B arc faults.
Briefly stated, and in accordance with a presently preferred embodiment of the invention, an arc fault detector for detecting electric power lines includes a sensor for sensing the derivative of the current on the electric power line, a converter circuit for converting the derivative of the line current into first and second signals, the first signal responsive to positive step transitions of arc fault current, and the second signal responsive to negative step transitions of arc current, and a temporal detector for signaling the presence of an arc fault when one of the first and second signals follows the other within a predetermined time, or window, and in which a sequence of one of the signals following the other signal occurs in a second predetermined interval of time.
In accordance with a preferred embodiment of the invention, the sensor is a current transformer with the electric power line being protected as its primary, and having a center tapped secondary winding producing first and second signal voltages of opposite polarity with respect to the center tap, in which the voltages are proportional to the rate of change of current in the power line primary winding, so that a step of current in the primary winding in one direction produces a pulse of voltage of one polarity, with respect to the center tap at one end of the secondary followed by a step of current in the other direction, which produces a second pulse of voltage of the same polarity with respect to the center tap at the other end of the secondary.
In accordance with another aspect of the invention, the current transformer has a core conformed of powdered metal.
In accordance with another aspect of the invention, the circuit for converting the derivative of the line current into first and second signals is an inverter circuit.
In accordance with another aspect of the invention, the first window of time is less than one half the period of the power line AC current.
In accordance with a still further aspect of the invention, a discriminator circuit is included for ignoring a predetermined number of signal sequences to reject normal load current step events. Brief Description of the Drawings:
The novel aspects of the invention are set forth with particularity in the appended claims. The invention itself, together with further objects and advantages thereof may be more readily comprehended by reference to the following detailed description of a presently preferred embodiment of the invention taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic diagram of an arc fault circuit interrupter in accordance with the invention for detecting series and parallel arcs;
FIG. 2 is a circuit diagram of an arc fault circuit interrupter in accordance with the invention for detecting series and parallel arcs in the presence of arc noise;
FIGS. 3 (a-j) form diagrams of signals appearing in the arc fault circuit interrupters of FIGS. 1 and 2;
FIG. 4 is a wave form diagram illustrating secondary ringing caused by high current parallel arc faults;
FIG. 5 is a schematic diagram of an arc fault circuit interrupter in accordance with an alternative embodiment of the invention, using a center tapped inductor as an arc sensor;
FIGS. 6 (a-b) is a schematic diagram showing alternate sensor circuit schematics for the circuit of FIG. 1; and
FIG. 7 is a schematic diagram of an arc fault circuit interrupter similar to that shown in FIG. 1, but including a parallel fault current by-pass circuit.